1. Field of the Invention
The present invention relates to a signal light transmitting apparatus which transmits signal light, a signal light transmitting system using the same, and a signal light transmitting method.
2. Description of the Related Art
Optimization of Power of Signal Light Which Enters Transmission Optical Fiber
In signal light transmitting system using transmission optical fiber, to secure a certain transmission quality, it is necessary that power of signal light which enters transmission optical fiber be greater than a predetermined lower limit. Signal light transmitted with transmission optical fiber is amplified by optical amplifier of repeater office. The lower limit is determined by amount of accumulation of optical noise (degree of depression of optical SNR) produced when amplifying signal light. The lower power of signal light which enters into transmission optical fiber from optical amplifier is, the more strongly signal light deteriorates.
It is known that transmission quality deteriorates through nonlinear optical effect occurring in transmission optical fiber during transmission of signal light. There are several types of phenomena which nonlinear optical effect causes. For example, there is phenomenon in which phase of signal light which propagates through transmission optical fiber is modulated. The phenomenon is caused by change of refractive index of transmission optical fiber depending on strength of power of signal light. GVD (Group Velocity Dispersion) of transmission optical fiber transforms phase modulation into intensity modulation to cause waveform distortion. The higher power of signal light in an optical fiber is, the more strongly the deterioration of signal light through nonlinear optical effect occurs. Accordingly, to secure a certain transmission quality, it is necessary that power of signal light which enters transmission optical fiber be less than a predetermined upper limit.
As mentioned above, it turns out that an optimum value of entrance power of signal light entering transmission optical fiber exists. Transmission quality of signal light transmitting system can be represented by Bit Error Rate (BER). When entrance power is too much, BER decreases through nonlinear optical effect. When entrance power is too little, optical SNR is depressed to decrease BER too. It is required that trade-off between both should be easily redressed.
Signal light transmitting system is designed so that gap between the upper limit and the lower limit of entrance power is enlarged enough. Optimum value in which BER is maximized exists between the upper limit and the lower limit. When value of entrance power is made higher than the optimum value, BER decreases. At this time, nonlinear optical effect occurs significantly. Alternatively, when entrance power is made less than the optimum value, optical SNR is depressed to decrease BER. Generally, value of entrance power is set at the value which does not exceed the optimum value, so that nonlinear optical effect may not occur significantly. That is, entrance power is set so that real value of entrance power is less than the value of entrance power, in which nonlinear optical effect occurs significantly, by a predetermined allowable value. During transmission of signal light, transmission loss which decreases power of signal light occurs. When signal light transmitting system is operated and maintained for a long period of time, transmission loss could increase and optical SNR could be depressed. For this reason, the allowable value is made as little as possible. Long-term deterioration of transmission optical fiber, increase in connection point of transmission optical fiber, and so on, increase transmission loss. The connection point of the transmission optical fiber is newly provided, when transmission optical fiber is cut in an accident.
Individual Optimization of Entrance Power
In a representative signal light transmitting system, several repeater offices and transmission optical fibers are used. Transmission line which transmits signal light is divided into several transmission sections by repeater offices. Optimum values of powers of signal lights which enter transmission optical fibers in transmission sections, respectively, are distinct from each other. There are two main reasons.
A variety of optical fibers is used for representative main transmission network. Probability of nonlinear optical effect occurring in optical fiber depends on parameters of the optical fiber, such as mode field diameter (core diameter) and amount of GeO2 doped in the optical fiber. When the type of the transmission optical fiber used for one transmission section differs from the one of the transmission optical fiber used for another transmission section, probabilities of nonlinear optical effect occurring in the transmission optical fibers differ from each other. This is the first reason. Parameters of optical fibers which distinct manufacturers manufacture are distinct from each other. Further, parameters of optical fibers manufactured at distinct manufacture periods are distinct from each other. Furthermore, the parameters of the optical fibers included in one manufacture lot differ from the parameters of the optical fibers included in another manufacture lot.
Loss in office building occurs in repeater office building. Optical connector is intermediate between optical amplifier and transmission optical fiber. Loss in office building is mainly caused in optical connector. In a large-scale repeater office, a large number of optical connectors are intermediate between the room in which the optical amplifier is installed and transmission optical fiber, and the loss in the office building could amount to several decibels. It is difficult to distinguish loss in office building and section loss caused between repeater offices. Loss in office building and section loss are treated as one.
Signal light which enters into transmission optical fiber from optical amplifier in office building is attenuated by loss in the office building. The phenomenon through the nonlinear optical effect which occurs when there is loss in office building occurs strongly compared with the case where it is assumed that there is not loss in office building. That is, it seems that probability of nonlinear optical effect occurring became lower. Because it is difficult to know loss in office building as mentioned above, it is difficult to know degrees of attenuations of signal lights caused in repeater offices, respectively. This is the second reason.
Jpn. Pat. No. 2856435 discloses the art of optimization of power of Raman pump light used for distributed Raman amplification. Loss distribution of transmission line is measured using OTDR, and power of Raman pump light is optimized so that ideal loss distribution is realized. Although optimum values of powers of pump lights transmitted in transmission sections are calculated, respectively, entrance powers of signal lights which enter optical fibers is fixed, respectively. Each of entrance powers of signal lights is kept at a certain constant ideal value. The state of each transmission section is not considered not to optimize each entrance power.
Jpn. Pat. KOKAI Publication No. 2001-251006 discloses the art of optimization of entrance power of signal light which enters optical fiber and power of Raman pump light used for distributed Raman amplification. The application on which U.S. Pat. No. 6,512,628 was based was filed based on Jpn. Pat. KOKAI Publication No. 2001-251006. The state of each transmission section is not considered not to optimize each entrance power, but each entrance power is adjusted so that the value of each entrance power become optimum value calculated beforehand. Additionally, the art of obtaining loss of the whole transmission line based on remains power of Raman pump light transmitted to opposite office is disclosed in the publication. Power of Raman pump light is adjusted in accordance with the loss of the whole transmission line. Although the loss of the whole transmission line can be obtained, localized loss, such as loss in office building is undistinguishable from the loss of the whole transmission line. Jpn. Pat. KOKAI Publication No. 10-233736 (filed based on the application on which U.S. Pat. No. 6,128,111 was based) discloses the art of optimization of entrance power of signal light entering transmission optical fiber based on amount of occurrence of Four-wave mixing (FWM) in transmission-line fiber. FWM is one of the phenomena caused through nonlinear optical effect. The amount of occurrence of FWM can be used as indicator for optimizing entrance power, because it is dependent on nonlinearity of transmission optical fiber. The amount of occurrence of FWM can be used as indicator indicates probability of nonlinear optical effect occurring, as long as phase matching condition, associated with frequency interval of light waves entering transmission optical fiber or GVD of transmission optical fiber, is satisfied and the polarization of each light wave takes the specified state. Entrance power can not be optimized unless phase matching condition is satisfied. When a large number of optical amplifiers are used, it is necessary to remove FWM light caused through FWM in each transmission section so that FWM light should not accumulate. When GVD of transmission optical fiber is higher, power of FWM light to cause becomes lower, and measurement of power of FWM light becomes difficult. For this reason, it is necessary to make entrance power extremely higher. It is difficult to optimize each entrance power.
Scattering Occurred in Optical Fiber
When light which enters optical fiber strikes the matter which the optical fiber is formed of, the light is dispersed and is said to be scattered. Scattering is classified into Rayleigh scattering, Raman scattering, Brillouin scattering, etc. depending on features of scattering. Rayleigh scattering originates from random density fluctuation fixed when glass gets cool and becomes solid. The scattered light and the entrance light have the same frequency.
Raman scattering is the scattering in which scattered light occurs by interaction of entrance light with molecular vibration or optical lattice vibration (optical phonon). Brillouin scattering is the scattering in which scattered light occurs by interaction of entrance light with acoustic lattice vibration (acoustic phonon). The difference between the frequency of the entrance light and the one of the scattered light (difference frequency) was decided. When Raman scattering or Brillouin scattering occurs, frequency of light shifts by the difference frequency (frequency shift). When power of incident light becomes higher, nonlinear coupling takes place between the entrance light and the scattered light. When the nonlinear coupling takes place, the lattice vibrates coherently at the difference frequency. It is known that stimulated emission of scattered light occurs at this time.
When stimulated Raman scattering occurs in optical fiber of silica glass, peak frequency which gives peak of spectrum of scattered light is lower than frequency of entrance light (pump light) by about 13 THz. Entrance light can be scattered in the forward direction in which entrance light propagates and the backward direction opposite to the forward direction. When stimulated Raman scattering occurs, peak frequency of scattered light is less than frequency of pump light by about 11 GHz. Entrance light is strongly scattered in only backward.
Stimulated Raman Scattering and Raman Amplification
When stimulated Raman scattering is used, light which enters optical fiber can be amplified. When signal light and Raman pump light which has higher frequency rather than frequency of signal light by about 13 THz simultaneously enter optical fiber of silica glass, portion of energy of the Raman pump light is transferred from the Raman pump light to the signal light via stimulated Raman scattering. As a result, signal light is amplified. This is called Raman amplification, and gain caused by Raman amplification is called Raman gain. Representative Raman gain has wavelength dependency as shown in FIG. 11. The transmission line formed from such optical fiber can be used as amplification medium (Distributed Raman Amplification).
Raman Gain Efficiency Measurement
Suppose that Raman gain (dB) occurs when pump light having a certain power (W) is input into optical fiber used as Raman amplification medium. Normalized variation of Raman gain (dB), obtained when changing pump light, by variation of power of pump light (W) is called Raman gain efficiency (dB/W).
Measurement of Raman gain efficiency is equivalent to measurement of Raman gain, when power of pump light is known. FIG. 12 shows a representative apparatus which measures Raman gain efficiency. The apparatus has devices 110, 120 and 510 for the measurement arranged at ends of the transmission line formed from optical fiber. Test light source 120 which emits test light is arranged on one end 312 side of transmission optical fiber 310 which connects two repeater offices 30. The wavelength of the test light is set up so that Raman gain efficiency is appropriately measured. Wavelength Division Multiplexing coupler (WDM coupler) 531 is arranged on the other end 311 side of transmission optical fiber 310. WDM coupler 531 can divide pump light and signal light, and WDM coupler 531 can make pump light and signal light overlap. Pump light source 110 is connected to pump wavelength range port 531a provided for WDM coupler 531, and measuring device 510 which measures the power of the test light amplified in transmission optical fiber 310 is connected to signal wavelength range port 531b provided for WDM coupler 531. In measurement of Raman gain efficiency, test light source 120, pump light source 110, and measuring device 510 are added to repeater office 30. It is necessary to modify optical system of repeater office 30 at the time of the addition.
First, the test light emitted by test light source 120 enters transmission optical fiber 310 at end 312, and power Pt1 (dBm) of the test light which is amplified in transmission optical fiber 310 and exits end 311 is measured. Pump light source 110 does not emit pump light during measurement of power Pt1. Subsequently, test light emitted by test light source 120 enters transmission optical fiber 310, with pump light emitted by pump light source 110, to measure power Pt2 (dBm) of the amplified test light. The emitted pump light enters transmission optical fiber 310 at end 311 via WDM coupler 531. Difference between Pt1 (dBm) and Pt2 (dBm) is Raman gain (dB) which test light obtains. When the power (W) of a pump light is changed, Raman gain (dB) changes. Normalizing variation of gain by variation of power of pump light, Raman gain efficiency (dB/W) is obtained.
Test light may enter transmission optical fiber 310 at end 311, test light may enter transmission optical fiber 310 at end 312, and pump light which is amplified in transmission optical fiber 310 and exits end 312 may be measured.
In the above-mentioned representative apparatus, it is necessary to operate at both ends of transmission line, and expensive test light source is required. Jpn. Pat. No. 3578343 to M. Sobe et al. discloses the art of measurement of power of Amplified Spontaneous Emission (ASE) light associated with Raman gain. According to the art, operation for the measurement is performed only on one end side of transmission line, without test light. It is not necessary to operate at both ends of transmission line.
Increase in Reflectance through Stimulated Brillouin Scattering
When pump light having narrow spectral line width enters optical fiber at one end, portion of the entering light is reflected in the optical fiber and the reflected light exits the one end. The reflected light includes the scattering light originating from backward Brillouin scattering. When the power of the entering pump light increases gradually and reaches a certain power Pc, reflectance (power of pump light/power of reflected light) can increase sharply (FIG. 13). This is increase in reflectance through stimulated Brillouin scattering. Frequency of light shifts slightly (by about 11 GHz).
Such phenomenon is disclosed in several publications, e.g., R. W. Tkach et al., “Spontaneous Brillouin Scattering for Single-mode optical-fibre characterisation”, Electronics Letters, vol. 22, no. 19, pp. 1011-1013, September 1986, which is incorporated herein by reference. According to the publication, amount of frequency shift changes in accordance with amount of GeO2 doped in optical fiber etc., and ranges from about 10.6 GHz to about 11.3 GHz.
As mentioned above, in order to acquire high transmission quality, it is desirable to make entrance power of signal light which enters transmission optical fiber higher as long as nonlinear optical effect dose not occur significantly. The entrance power in which nonlinear optical effect starts to occur significantly in one transmission section differs from that in which nonlinear optical effect starts to occur significantly in another transmission section. The art of obtaining each entrance power easily is not known.
The art in which light is observed only in terminal office to estimate transmission quality is known. However, when probabilities of nonlinear optical effect occurring in transmission sections are distinct from each other, it is difficult to obtain the entrance power for each transmission section in which nonlinear optical effect starts to occur significantly, from the observation result obtained only in terminal office.
Generally, when nonlinear optical effect is likely to occur (i.e., when the entrance power in which nonlinear optical effect starts to occur significantly is low), entrance power is set up lower. When probability of nonlinear optical effect occurring is unknown, entrance power is set up relatively lower. As long as the entrance power is set up relatively lower, when real probability of nonlinear optical effect occurring is higher, nonlinear optical effect is not likely to occur.
As mentioned above, it is difficult to know degrees of attenuations of signal lights caused from loss in repeater office buildings, respectively. In order to compensate the attenuation caused from loss in office building, output power of signal light which optical amplifier outputs is enlarged by amount of the attenuation. When loss in office building is low, output power is reduced. When loss in office building is unknown, output power is set up relatively lower. In the case that output power is set up relatively low, when real loss in office building is lower, it is prevented that entrance power of signal light which enters transmission optical fiber becomes unexpectedly high and the entrance power exceeds the power in which nonlinear optical effect starts to occur significantly.
Accordingly, when value of entrance power in which nonlinear optical effect starts to occur significantly and loss in office building are unknown, output power of signal light which optical amplifier outputs must be low.